1. Field of the Invention
The present invention relates to an optical semiconductor memory device and a method for reading/writing from/into the optical semiconductor memory device. More specifically, the invention relates to an optical semiconductor memory device known as a wavelength-domain-multiplication memory utilizing quantum dots, and a method for reading/writing information from/into the optical semiconductor memory device.
2. Description of the Prior Art
With the enormous progress in information communication technology, it is highly desirable to build a system which makes it possible to make efficient use of respective features of an optical device and an electronic device.
Features of optical devices are higher speed processing and parallel processing, when compared to the electronic devices. Up to this time, various storage devices (memory devices) using optical devices have been proposed, but none have been embodied.
Considering a state of the art wherein peripheral logic circuits and/or switching circuits are consist of electronic devices, even if the memory cells are built by the optical devices, it would be important to make the best use of the benefits and features of respective electronic devices and optical devices. In existing technology, no clear device concept of optical memory has been provided.
To pursue a structure enabling the semiconductor to be a lower order of magnitude may lead to a quantum dot. Such quantum dot would be attractive in physical aspects since its state density is discrete in the quantum dot.
The quantum dots have been expected as optical memories since transition energy between ground levels in the quantum dots (quantum boxes) has extremely steep optical absorption spectra. Furthermore, it becomes feasible to read/write information by light irradiation if optical absorption saturation is strong.
In the event that shapes or sizes of the quantum dots are changed, absorption wavelength may be changed for every quantum dot. Therefore it becomes possible to store information in respective quantum dots by multiple wavelength light irradiation. Assuming that one-bit information can be stored in every quantum dot, one-bit information can be stored in a region of 10 nm .times.10 nm and thus one-terabit information can be stored in a region of 1 cm.sup.2.
Although satisfactory results have not been achieved due to the fact that quantum dots are difficult to form, several reports for forming the quantum dots have been offered in recent years.
For the purposes of example, if a quantum well structure can be fabricated by MBE method wherein plural-layered InGaAs layer having an In composition of about 0.5 and largely different lattice constants on a GaAs substrate, quantum dots of a 30 to 40 nm diameter could be formed. This has been reported in literature, such as D. Leonard et al., Appl. Phys. Lett. 63 (23), 1993, pp. 3203-3205.
Further, quantum dots of a 15 to 20 nm diameter could be formed at a distance of about 100 nm by atomic layer epitaxy (ALE) growth technique. This has been recited in literature, such as Kohki Mukai et al., Jan. J. Appl. Phys. Vol. 33 (1994), pp. L1710-L1712. The fact that similar quantum dots like the above could be obtained by MOVPE has been reported in literature, such as J. Oshinowo et al., Appl. Phys. Lett. 65 (11), 1994, pp. 1421-1423.
Furthermore, a structure has been reported by A. Kurtenbach et al., Appl. Phys. Lett. 66 (3), 1995, pp. 361-363, wherein InP quantum dots can be buried between InGaP layers by supplying InP on a first In.sub.x Ga.sub.1-x P (x=0.49) layer being grown on a GaAs substrate and then forming a second In.sub.x Ga.sub.1-x P (x=0.49) thereon.
It can be considered that the quantum dots formed as discussed above could be formed by positively causing distortion in crystal in growth process (Stranski-Krastanow mode).
One of requirements for the wavelength multiplication optical memory will be to achieve a wideband optical absorption material.
In the above-discussed literature, since the quantum dots can be formed in a self-organized fashion, they are subjected to thermal fluctuation, so that fluctuation in composition and fluctuation in size are caused. It would be understood that fluctuation in size is dominant in various fluctuation.
The magnitude of fluctuation in size of the quantum dots is such as 80 meV if represented by full-width at half maximum (FWHM) of photoluminescence. This is more than ten times as large as FWHM of two-dimensional quantum well layer. The wavelength multiplication optical memory may be characterized by utilizing this fluctuation positively.
However, it will be limited to increase FWHM by using this fluctuation, and it is impossible to increase FWHM more than 100 meV.
A structure and operation of the wavelength multiplication optical memory will be as follows.
As shown in FIGS. 1A and 1B, in the wavelength-domain-multiplication memory, a buffer layer 202 formed of AlGaAs may be formed on a (001) face of a GaAs substrate 201, and a plurality of quantum dots 204 formed of InGaAs put between two barrier layers 203, 205 formed of GaAs may be formed thereon. If the quantum dots 204 are formed of InP, two barrier layers 203, 205 are formed of GaAs and the buffer layer 202 are formed of GaAs.
In a wavelength-domain-multiplication memory having this structure, if the light of specific wavelength is irradiated, the hole-electron pairs are created by photo-excitation in the quantum dots 204, as shown in FIG. 1C. Since the electrons have high tunneling probability and X point band energy of AlAs is low, the electrons can pass through the AlGaAs barrier to move to exterior of the quantum dots. Transition of the electrons may be detected as a change in the current.
If electric field E is applied in the film thickness direction upon light irradiation, the electrons may flow from the quantum dots 204 because of tunneling, etc. and move in the electric field E; therefore, transition of the electrons may be measured as change in the current. Conversely, since holes cannot pass through the barrier due to a large effective mass, they stay in the quantum dots 204. As a result, information "1" has been written, for example, in the quantum dots 204.
Moreover, if the light of a specific wavelength is irradiated to the quantum dots 204 to read information, the electrons do not flow out of the quantum dots 204, since only the holes reside in the quantum dots 204. Consequently, a change in the current does not occur, and in that case data "1" has been read out.
On the other hand, it can be regarded as writing of information "0" not to create the hole-electron pairs in the quantum dots 204 without initial light irradiation to the quantum dots 204. Then, if the light of specific wavelength is irradiated to read information while applying electric field in the film thickness direction, the hole-electron pairs are created by photo-excitation in the quantum dots 204, which can be measured as change in the current. In this event, data "0" has been read out.
The wavelength-domain-multiplication memory is such a storage device that plural kinds of quantum dots having different reading/writing wavelengths are provided. For example, the wavelength-domain-multiplication memory has been discussed, for example, by Shunichi Muto, Jan J. Appl. Phys. Vol. 34 (1995), pp. L210-L212.
The wavelength-domain-multiplication memory disclosed in this literature comprises an AlAs substrate having an inclined principal face and a group III-IV semiconductor layer. On the principal face of the AlAs substrate, steps are formed in two directions according to the inclination. GaAs's are formed at the corners of the steps, and an AlGaAs layers are formed around the corners. Thus GaAs's surrounded by the AlGaAs layers may serve as the quantum dots (quantum boxes).
Conversely, since the holes are difficult to flow out of the quantum dots, the holes remain in the quantum dots. Under this condition, even if the lights having the same wavelength are irradiated to the quantum dots, light absorption is seldom caused since the quantum dots occur strong absorption saturation due to remaining holes.
It would be feasible to read/write information from or into the quantum dots by using such a phenomenon. If plural quantum dots having different sizes and shapes were utilized, different reading/writing wavelengths may be employed quantum dot by quantum dot, so that a significant amount of information can be stored by multiple wavelengths. Thus, the wavelength multiplication memory has been accomplished.
However, since there is a high probability that the electrons being emitted from the quantum dots may return to the quantum dots to recombine with the holes therein, a retaining time of this wavelength multiplication memory is reduced, such as 1 to 10 ms. Since it is mandatory to detect by optical absorption whether or not absorption saturation appears in the quantum dots, a practical structure of the memory has not yet been taught.